I commented out the following lines from the code:
'Dim files As List(Of HttpPostedFile) = DirectCast(Cache(Me.Key), List(Of HttpPostedFile))
'For Each file As HttpPostedFile In files
'mailMessage.Attachments.Add(New Attachment(file.InputStream, Path.GetFileName(file.FileName), file.ContentType))
'Next
instead I used the following line:
mailMessage.Attachments.Add(New Attachment(Server.MapPath("~/Images/2006 BMW X5 Title.pdf")))
and the email goes with the PDF attachment.
My thought is that the issue is with the following line, when trying to send anything other than an HTML and txt file:
mailMessage.Attachments.Add(New Attachment(file.InputStream, Path.GetFileName(file.FileName), file.ContentType))
Hope this helps.